The present invention relates generally to the art of electronic packaging and more specifically relates to methods of making compliant semiconductor chip packages.
Modern electronic devices utilize semiconductor chips, commonly referred to as xe2x80x9cintegrated circuitsxe2x80x9d that incorporate numerous electronic elements. These chips are typically mounted on substrates, such as printed circuit boards, which physically support the chips and electrically interconnect each chip with other elements of the circuit. The substrate may be a part of a discrete chip package used to interconnect a single chip to external circuits or may be a xe2x80x9cmodulexe2x80x9d whereby one or more chips are mounted directly to a substrate which interconnects the chips with other circuit elements mounted to the substrate. In either case, the semiconductor chip(s) must be securely assembled with the substrate and must have reliable electrical interconnection(s) to the substrate.
Advanced semiconductor chips may require hundreds of input/output (xe2x80x9cI/Oxe2x80x9d) connections and the substrate must accommodate all of the required external electrical interconnections to the chip. Structures connecting the chip to the substrate ordinarily are subject to substantial strains caused by thermal cycling as temperatures within the chip package change during operation. Typically, the chip and the substrate expand and contract by different amounts. This causes the electrical contacts on the chip to move relative to the electrical contact pads on the substrate, thus deforming the electrical interconnections between the chip and substrate and placing them under mechanical stress. These repeated stresses can cause breakage of the electrical interconnections.
U.S. Pat. Nos. 5,148,265 and 5,148,266, the disclosures of which are hereby incorporated by reference herein, solve these problems by providing a flexible, sheet-like interposer including conductive terminals and flexible leads connected to and extending from the terminals. This flexible layer is mounted over the face of a semiconductor chip, preferably with a soft, compliant material disposed beneath the flexible layer and the terminals. The conductive terminals are electrically connected to electrical contacts on the semiconductor chip by the flexible leads. The conductive terminals can be connected or bonded to contact pads on a substrate so as to connect the semiconductor chip to the substrate. Because the terminals are moveable with respect to the contacts on the semiconductor chip, the assembly compensates for thermal expansion. Also, because the terminals are compliant or moveable in the vertical directions normal to the face of the chip, the terminals can be readily engaged with a test probe before assembly to the substrate. Thus, the subassembly can be tested prior to assembly to the substrate.
Commonly assigned U.S. Pat. No. 5,548,091, the disclosure of which is hereby incorporated by reference herein, discloses a prefabricated interposer or connection component for a semiconductor chip. The connection component includes a flexible dielectric film having top and bottom surfaces and further includes conductive terminals accessible at a surface of the dielectric film and flexible leads extending from the terminals. The connection component further includes an adhesive disposed on the bottom surface of the flexible dielectric film for bonding the bottom surface of the dielectric film to the semiconductor chip. The adhesive desirably is solid and non-tacky at temperatures below a preset activation temperature, but is adapted to reach a flowable condition upon heating to above the preset activation temperature, and to form a bond after such heating. Preferably, the adhesive is adapted to form a relatively weak bond to the bottom surface of the dielectric film or to the surface of the chip. Thus, the connection component can be removed from a chip after bonding thereto. This greatly facilitates repair and reclaim of chips from subassemblies which prove to be defective when tested. Such defective subassemblies may arise, for example, where there is a fault in the connection component or the bonding process.
For example, commonly assigned U.S. Pat. No. 5,659,952, the disclosure of which is hereby incorporated by reference herein, discloses a method of fabricating a compliant interface for a semiconductor chip typically comprised of a compliant encapsulation layer having a controlled thickness. In certain preferred embodiments of the ""952 Patent, a connection component, such as a flexible, substantially inextensible dielectric film, is provided. A compliant element, such as a plurality of compliant pads defining channels therebetween, is attached to a first surface of the first support structure. The compliant pad/connection component subassembly is then assembled with a semiconductor chip having a front face including a plurality of contacts. During assembly, the front face of the semiconductor chip is abutted against the compliant pads and the contacts are electrically connected to corresponding terminals on a second side of the dielectric film. A curable liquid encapsulant material, such as a curable silicone elastomer, is then provided between the semiconductor chip and the dielectric film and around the compliant pads while the chip and the dielectric film are held in place. The liquid encapsulant is then cured, whereby the compliant pads and the cured encapsulant provide a substantially continuous compliant interface between the semiconductor chip and the flexible dielectric film.
The above-referenced ""697 application discloses a method of encapsulating a semiconductor chip package. According to preferred embodiments of the ""697 application, a semiconductor chip package assembly has a compliant element or spacer layer between the top surface of a flexible dielectric film and the contact bearing surface of a semiconductor chip. The flexible dielectric film has conductive leads thereon, the leads having first ends which are electrically connected to the terminals and second ends which are bonded to the respective chip contacts. A protective layer is attached on a bottom surface of the dielectric film to cover the terminals on the substrate and to seal any apertures in the dielectric film. After attachment of the protective layer, a flowable, curable encapsulant material is deposited around at least a portion of a periphery of the semiconductor chip so as to encapsulate the conductive leads. The protective layer prevents the curable encapsulant from flowing through any dielectric film apertures. The encapsulant material is then cured or at least partially cured to allow for handling and/or further processing.
One aspect of the present invention provides a method of making a microelectronic package. The method includes providing a first microelectronic element having electrically conductive parts and including a first surface and a second surface. The first microelectronic element preferably includes a flexible dielectric film having conductive terminals accessible at one or more surfaces thereof and flexible leads integrally connected to the conductive terminals and extending therefrom. A compliant element, such as a silicone elastomer, is preferably provided over the first surface of the first microelectronic element. The combination of the first microelectronic element and the compliant element is hereinafter referred to as the xe2x80x9cpackage subassembly.xe2x80x9d The compliant element preferably includes a xe2x80x9creleasable adhesive,xe2x80x9d i.e., an adhesive material which is capable of at least temporarily securing two or more microelectronic elements together so that the elements will remain together, but so that the elements can be separated from one another deliberately without destroying the elements.
In certain preferred embodiments, a second microelectronic element having electrically conductive parts is provided. The second microelectronic element preferably includes a semiconductor chip having a front face including electrical contacts on a peripheral region of the front face. The second microelectronic element is abutted against the releasable adhesive so that the second microelectronic element is releasably assembled to the first microelectronic element. For example, when the second microelectronic element is a semiconductor chip, the chip is preferably assembled with the package subassembly by abutting the front face of the semiconductor chip against the releasable adhesive. Thus, the first and second microelectronic elements form a releasably assembled package. The package may be tested and evaluated before the elements are permanently assembled together. If the releasably assembled chip package fails any one of a number of tests then the releasable adhesive allows the relatively expensive microelectronic elements of the package, such as a semiconductor chip, to be easily disassembled from the package and reassembled with other microelectronic elements or package subassemblies. The bond created by the releasable adhesive is relatively weak so that the chip can be removed from the package subassembly without destroying the chip or rendering the chip useless. The releasable adhesive may include a material selected from the group consisting of pressure-sensitive adhesives, thermoplastics and polyimide siloxane adhesives. In certain embodiments, the compliant element may completely comprise the releasable adhesive. However, in other embodiments the compliant element may include a core or central region including a compliant material, such as a silicone elastomer, and one or more exterior surface regions including the releasable adhesive. Most preferably the releasable adhesive is provided at a surface of the compliant element which is remote from the first microelectronic element. The remote surface of the compliant element is further defined below as the surface which is preferably abutted against the second microelectronic element.
After the first microelectronic element and the compliant element including the releasable adhesive have been assembled together, the resulting package subassembly may be stored prior to assembly with the chip or other second microelectronic element. The package subassembly is preferably prepared for storage by providing a release liner, such as a relatively thin, flexible plastic sheet, over the releasable adhesive at the remote surface of the compliant element. The release liner may include a release treatment such as the synthetic flourine-containing resin commonly sold under the trademark Teflon(copyright), at one or more surfaces thereof, so that, when necessary, the release liner may be readily removed from the compliant element to expose the releasable adhesive. Package subassemblies manufactured according to this particular preferred embodiment may be fabricated at one location, placed in storage and then shipped to remote manufacturing facilities for chip or die attach processes.
In certain preferred embodiments, after the first and second microelectronic elements have been releasably assembled together, the elements are electrically connected to one another by bonding the flexible leads of the first microelectronic element to the electrical contacts of the second microelectronic element. The microelectronic package is then tested and evaluated to determine whether the package is operating properly and to uncover any damage and/or defects in the package. Common defects typically include misalignment of the contacts of the second microelectronic element with respect to the flexible leads of the first microelectronic element and/or misalignment of the compliant element over the first surface of the first microelectronic element. As such, one portion of the test may include an inspection to assure that the electrically conductive parts of the first and second microelectronic elements are in substantial alignment with one another. Another portion of the test may include engaging the electrically conductive parts of the first microelectronic element with a test probe to determine whether the electrically conductive parts of the first and second microelectronic elements have been properly interconnected. If testing uncovers a defect in the package after the leads have been bonded to the chip contacts, the flexible leads may be broken and the lead remnants/package subassembly removed so that the second microelectronic element may be reused on another package subassembly. The testing step may precede the step of electrically interconnecting the first and second microelectronic elements or may be conducted after the first and second microelectronic elements have been electrically interconnected.
The utiliation of a releasable adhesive for releasably assembling the second microelectronic element with the package subassembly makes it possible to test and evaluate the operability of the package before the second microelectronic assembly is permanently assembled to the package subassembly. As a result, a manufacturer may selectively remove or disassemble the second microelectronic element from the microelectronic package subassembly if any defects are discovered during testing and evaluation of the packages. As a result, operationally sound microelectronic elements (i.e. xe2x80x9cgoodxe2x80x9d semiconductor chips) may be reclaimed from defective packages without destroying the integrity of the chips so that the chips may be reassembled with other fully operational package subassemblies. This particular benefit can result in substantial cost savings because complex microelectronic elements, such as semiconductor chips, are relatively expensive in comparison to the other elements of the microelectronic package (e.g. the flexible dielectric film).
After the releasably assembled package has passed the tests described above, the first and second microelectronic elements may be permanently assembled together by using a xe2x80x9clock-downxe2x80x9d encapsulant. The xe2x80x9clock-downxe2x80x9d encapsulant preferably includes a curable liquid material such as a curable silicone elastomer. Before the liquid encapsulant is introduced, a protective coverlay is preferably provided over the second or exterior surface of the first microelectronic elements to isolate and protect the conductive terminals from the curable liquid encapsulant so that the terminals are not covered by the curable liquid and remain accessible after this stage of the assembly process. The curable liquid encapsulant is then provided between the first and second microelectronic elements and around the compliant element. The liquid encapsulant preferably surrounds the compliant element and those portions of the electrically conductive parts lying between the first and second microelectronic elements. In further preferred embodiments, a second protective coverlay may be provided over the rear surface of the second microelectronic element to prevent the curable liquid encapsulant from coming in contact with the rear surface thereof. The second protective coverlay is preferably substantially similar to the first protective coverlay. The curable liquid encapsulant may then be cured to permanently assemble the first and second microelectronic elements. The cured liquid encapsulant and the compliant element preferably provide a continuous, uniform, compliant layer between the first and second microelectronic elements. The cured encapsulant and the compliant element are preferably CTE matched and comprise materials which are substantially similar, i.e. silicone elastomer.
After the curable liquid encapsulant has been cured, the cured encapsulant layer may be severed outside the perimeter of the second microelectronic element to provide an individual microelectronic package having a compliant bumper. The protective coverlays may be maintained in place while the package is in storage to protect the package from contamination. Preferably, the protective coverlay over the exterior surface of the first microelectronic element is maintained in place while the package is in storage in order to isolate the conductive terminals from contaminants. When it is desirable to electrically connect the permanently assembled microelectronic package with an external circuit element, such as a printed circuit board, the protective coverlay may be removed from the exterior surface of the first microelectronic element to expose the conductive terminals so that the conductive terminals may be electrically interconnected with contacts on the external circuit element.
In other preferred embodiments, the compliant element may include a plurality or array of compliant pads, whereby the compliant pads define channels running between any two adjacent pads. The compliant pads preferably include the releasably adhesive, most preferably at one or more surface regions thereof. When the curable liquid encapsulant is provided between the first and second microelectronic elements, the liquid encapsulant flows through the channels between the compliant pads. After the liquid encapsulant has been cured, the cured encapsulant and the compliant pads provide a continuous and uniform compliant layer between the first and second microelectronic elements.
In other preferred embodiments of the present invention, a method of making a plurality of microelectronic packages includes providing a plurality of first microelectronic elements, such as the flexible dielectric sheet described above, having electrically conductive parts and including first and second surfaces. A plurality of compliant elements are then provided over the first surfaces of the first microelectronic elements. Each compliant element includes a releasable adhesive at a surface remote from the first microelectronic element. A plurality of second microelectronic elements, such as the semiconductor chips described above, are then provided and abutted against the releasable adhesive so that the second microelectronic elements contact the releasable adhesive of the compliant element and are releasably assembled with the first microelectronic elements. The first and second microelectronic elements are then electrically interconnected with one another. The resulting plurality of microelectronic packages are then tested and evaluated using the techniques described above. As mentioned above, testing of the packages may occur before or after the plurality of first and second microelectronic elements have been electrically interconnected with one another, or both before and after such electrical interconnection. During testing, the releasably assembled packages are evaluated to identify properly and improperly assembled packages. Preferably, the first and second microelectronic elements of any improperly assembled packages are disassembled so that the second microelectronic elements may be reclaimed. The reclaimed second microelectronic elements may then be reassembled with other package subassemblies to provide reassembled microelectronic packages. The reassembled packages are then electrically interconnected and re-tested to determine whether they are properly or improperly assembled. A curable liquid is then provided between the first and second microelectronic elements of the properly assembled packaged and the curable liquid is cured so that the first and second microelectronic elements are permanently assembled to one another.
These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiment set forth below and when taken in conjunction with the accompanying drawings.